The growth of living cells in a stirred liquid culture medium is well established and is important both in the research and industrial fields.
Some cell types can be grown in a stirred suspension culture without the need for the presence of an attachment surface. However, certain cell types exist which can grow only in the suspension by being termed anchorage-dependent.
In recent years methods for growing anchorage-dependent cells in suspension cultures have been developed using microcarriers, see for example U.S. Pat. No. 4,036,693. Thus use of such microcarriers is more economic relative to other known methods as it provides a large surface area for growth in relation to the volume of culture.
Normal methods for cell growth in a liquid culture medium containing microcarriers involve the necessity to maintain the microcarriers in uniform suspension while avoiding damage to growing anchorage-dependent cells.
One accepted method of stirring cell cultures has been a horizontally revolving permanent magnet either suspended by some form of bearing or as a free component allowed to find its own position in a flat bottomed culture flask. One such device is described in U.S. Pat. No. 3,572,651. Such devices are, however, not satisfactory for stirring microcarrier cultures as their minimum threshold speed is high enough to cause damage to the cells and vortices and stagnant zones are created within the stirred medium. In particular a point of stagnation develops immediately below the axis of rotation of the magnet leading to conditions of uncontrolled stirring within the culture medium and consequent non-uniformity of suspension of the microcarriers. Moreover the amount of heat generated by the motor driving the magnet is so great as to render the equipment unsuitable for use in an incubator without employing refrigeration.
A superior type of stirrer for microcarriers suspended in a liquid culture medium which obviates this problem, is disclosed in U.K. Pat. No. 1,485,741. Such a stirrer is basically comprised of a stirring rod for immersion in a liquid to be stirred contained in a flat bottomed cylindrical culture flask, the stirring rod being provided with means for imparting to it an oscillatory pivotal motion such that the end of the rod describes an orbital path within the culture flask.
One disadvantage of this type of stirrer, as with any stirrer designed to impart a rotational primary movement to a culture liquid within a flat bottomed cylindrical vessel, is that due to the effects of secondary motion within the liquid an accumulation of suspension particles occurs at the outer perimeter and the centre of the bottom of the culture vessel.
The explanation for this accumulation is well known. For example reference may be made to "The cause and formation of meanders in the course of rivers" and the so called "Beers Law" from "Ideas & Opinions" by Albert Einstein (Alvin & Redway Ltd, 1954).
In brief, the rotation of the liquid medium during the stirring action, which may be termed the primary motion of circulation, causes a centrifugal force to act on it. At the walls of the stirring vessel the liquid is restrained by friction, so that the angular velocity at which it circulates is less at the walls than in other places near the centre. In particular the angular velocity of circulation of the liquid medium, and hence the centrifugal force will be smaller near the bottom of the vessel than higher up.
The net result will be a vertical circulating movement or secondary circulating motion as opposed to the horizontal or primary circulating motion, of the liquid medium in the vessel. The suspension particles are swept by the secondary motion into the centre and corners of the bottom of the vessel, which effectively form stagnant zones during stirring within the vessel, and accumulate there with time.